1. Field of the Invention
The present invention relates to the field of gain control circuits for power amplifiers.
2. Prior Art
In certain applications, power amplifiers are provided with a gain control capability so that the output of the power amplifier may be automatically controlled or changed as desired. By way of example, in wireless RF telephone devices such as in cellular systems, the broadcast power of the mobile unit is controlled by the base station to provide an adequate, but not excessive, received signal strength at the base station for recovery of the information in the received signal. Accordingly, in the mobile unit, the gain of the RF power amplifier is controlled by a gain control feedback loop holding the power output of the power amplifier to the power level commanded by a signal received from the base station. In such gain control feedback loops, it is normally desired to provide a feedback signal which is linear with the power output in dBm of the power amplifier. This aids in maintaining a constant bandwidth of the gain control loop, making the loop easier to stabilize and in providing a uniform response throughout its operating range, and in providing a more accurate response to a commanded change in power amplifier output received from the base station.
In a typical wireless device, the RF output of the power amplifier is capacitively coupled to a detector circuit which is intended to detect power level in the RF signal and to provide an appropriate gain control feedback signal in response thereto. The typical prior art detector utilizes a diode circuit to provide a peak detector on the RF signal, utilizing the non-linear characteristics of the diode to obtain the detector response. That response, however, while approximating a response linear in dBm with power amplifier output for the lower power outputs of the power amplifier, curves upward at the higher power amplifier outputs to grossly deviate from the desired linear characteristic. This is illustrated in the upper curve in FIG. 1. As a result, either analog or digital compensation techniques for this effect must be included in the circuitry, or alternatively, compromises in stability and accuracy of commanded gain changes must be tolerated. Further, diodes as used for peak detection are temperature sensitive, and the low battery voltages now commonly being used severely limit the headroom of the detector circuit, further limiting the performance of typical prior art detectors.